In an axial turbine of a steam turbine or a gas turbine applied, for example, to a power plant, there have recently been reviewed improvement in thermal efficiency, and especially, improvement in a turbine internal efficiency, by which an economic operation can be carried out effectively.
A subject to suppress the secondary flow loss due to the secondary flow of working fluid such as working steam or working gas in a turbine nozzle unit or a turbine movable blade unit, of losses including a blade profile loss occurring in a turbine blade and the secondary flow loss (secondary loss) of the working fluid, as low as possible, in order to improve remarkably the turbine internal efficiency, has been addressed as one of significant subjects of study.
FIG. 10 is a view illustrating a structure of a turbine nozzle unit called the “straight blade”, which is conventionally applied to the axial turbine. A plurality of nozzle blades 1 (so called the “stationary blades”) is placed in a row in a circumferential direction of a turbine axis, not shown, of an annular passage 4, which is formed between an outer diaphragm ring 2 and an inner diaphragm ring 3.
A plurality of turbine movable blades 5 is placed in the circumferential direction on the downstream side of the nozzle blades 1, so as to correspond to the row arrangement of the nozzle blades 1, as shown in FIG. 8. The turbine movable blades 5 are implanted in a rotor disc 6 in the peripheral direction thereof and are provided at the respective outer peripheral ends with a shroud 7, which prevents the working steam or the working gas (hereinafter referred to as the “working fluid main stream” or merely to as the “main stream”) from leaking.
Detailed description will be given below of a mechanism of occurrence of the secondary flow of the working fluid on the nozzle blade 1 (hereinafter referred merely to as the “secondary flow”) in the axial turbine having the above-described structure, with reference to FIG. 10, which is a perspective view, in which the turbine nozzle unit is viewed from the outlet side of the nozzle blade 1.
The working fluid main stream flows the passage between the blades in a curved shape. At this stage, a centrifugal force is generated from the back (dorsal) side “B” of the nozzle blade 1 toward the front (ventral) side “F”. The centrifugal force is balanced with static pressure so that the static pressure on the front side “F” becomes higher.
On the contrary, the flow velocity of the main stream is high on the back side “B”, resulting in the lower static pressure. This causes a pressure gradient to occur from the front side “F” towards the back side “B” in the passage between the blades. The pressure gradient also occurs in a boundary zone formed on the peripheral wall surface of the outer diaphragm ring 2 and the inner diaphragm ring 3 in the similar manner.
However, the flow velocity is low and the centrifugal force becomes small in the boundary zone in the passage between the blades, with the result that endurance against the pressure gradient from the front side “F” towards the back side “B” cannot be maintained, thus producing the secondary flow 8 of the working fluid, which is directed from the front side “F” toward the back side “B”.
The secondary flow 8 collides with the back side “B” of the nozzle blade 1 to rise up, thus producing the secondary flow vortexes 9a, 9b in connection portions at which the nozzle blade 1 is connected to the outer diaphragm ring 2 and the inner diaphragm ring 3 so as to support the nozzle blade 1.
The energy possessed by the main stream of the working fluid is lost partially under the influence of development and diffusion of the secondary flow vortexes 9a, 9b, and the wall friction due to the secondary flow, in this manner, thus becoming a factor responsible for the remarkably deteriorated turbine internal efficiency. The secondary flow loss also occurs in the turbine movable blade unit in the same manner as the turbine nozzle unit.
There have been disclosed many results of research and many proposals to reduce the secondary flow loss due to the secondary flow vortexes 9a, 9b, which are generated in the passage between the blades.
There has been disclosed for example a turbine nozzle unit, which has a profile in which a throat-pitch ratio “s/t” expressed by a throat “s”, which is defined by the shortest distance between the rear edge of a nozzle blade 1 and the back side “B” of another nozzle blade 1 that is adjacent to the above-mentioned nozzle blade 1, and a pitch “t” of the blades 1 aligned annularly, is maximized at a blade-central portion in height, on the one hand, and decreased at the blade-root portion and the blade-tip portion, on the other hand, as shown in FIG. 9 (see Japanese Laid-Open Patent Publication No. HEI 6-272504).
The above-mentioned turbine nozzle unit has advantages as described below in comparison with a turbine nozzle unit or turbine movable blade unit, which has conventionally been applied for example to a steam turbine and called the “straight blade” type (i.e., the blades placed along the radial lines, which pass through the center of the turbine axis and straightly extend radially). In the turbine nozzle unit called the “straight blade” type, the loss at the blade-central portion in height is small, on the one hand, and the loss at the blade-root portion and the blade-tip portion becomes relatively large, on the other hand, as shown in FIG. 5A. Furthermore, in the turbine movable blade unit called the “straight blade” type, the loss at the blade-central portion in height is small, on the one hand, and the loss at the blade-root portion and the blade-tip portion becomes relatively large, on the other hand, as shown in FIG. 5B. The “loss” means loss of the secondary flow of the working fluid in the following description, unless a definition is specifically given.
On the contrary, in the turbine nozzle unit having the profile in which the throat-pitch ratio “s/t” is maximized at the blade-central portion in height, on the one hand, and decreased at the blade-root portion and the blade-tip portion, on the other hand, as shown in a dotted line in FIG. 4A, the flow rate of the main stream is decreased at the blade-root portion and the blade-tip portion in which the larger loss occurs, on the one hand, and increased at the blade-central portion in height in which the smaller loss occurs, on the other hand. Accordingly, the loss generated in the whole passage in the turbine nozzle unit becomes smaller in comparison with the turbine nozzle unit called the “straight blade” type.
Furthermore, in the turbine movable blade unit having the profile in which the throat-pitch ratio “s/t” is maximized at the blade-central portion in height, on the one hand, and decreased at the blade-root portion and the blade-tip portion, on the other hand, as shown in a dotted line in FIG. 4B, the loss generated in the whole passage in the turbine movable blade unit becomes smaller in comparison with the turbine movable blade unit called the “straight blade” type, in the same manner as the above-described turbine nozzle unit.
In addition, with respect to the other results of research, there has been disclosed a turbine nozzle unit called “compound lean” type in which the nozzle blades 1 bend relative to the radial lines, which pass through the center of the turbine axis (which is indicated by the reference sign “E” in FIG. 10) (see Japanese Laid-Open Patent Publication No. HEI 1-106903).
The turbine nozzle unit called the “compound lean” type has a structure as shown in FIG. 7A in which the rear edge of the blade projects in a curved profile from the blade-tip portion and the blade-root portion towards the blade-central portion in height so as to generate pressing forces, which are applied from the blade-tip portion and the blade-root portion to the outer and inner diaphragm rings 2 and 3, respectively. Accordingly, the turbine nozzle unit called the “compound lean” type makes it possible to keep the small pressure gradient in the boundary zone generated in each of the outer diaphragm ring 2 and the inner diaphragm ring 3.
The turbine movable blade unit also has a structure as shown in FIG. 7B in which the rear edge of the blade projects in a curved profile from the blade-tip portion and the blade-root portion towards the blade-central portion in height so as to generate pressing forces, which are applied from the blade-tip portion and the blade-root portion to a shroud 7 and a rotor disc 6, respectively, in the same manner as the above-described turbine nozzle unit, thus making it possible to keep the small pressure gradient in the boundary zone generated in each of the shroud 7 and the rotor disc 6 (see Japanese Laid-Open Patent Publication No. HEI 3-189303).
The turbine nozzle unit and the turbine movable blade units, which are called the “compound lean” type, have the profile by which the pressing force applied from the blade-tip portion to the outer diaphragm ring 2 as well as the pressing force applied from the blade-root portion to the inner diaphragm ring 3 are given, and the pressure gradient in the boundary zone generated in each of the outer diaphragm ring 2 and the inner diaphragm ring 3 is kept small, thus leading to a larger flowing amount of the main stream.
However, the connection portion of the blade-tip portion to the outer diaphragm 2 and the connection portion of the blade-root portion to the inner diaphragm 3 originally exist as zones where the secondary flow loss of the working fluid is large. Accordingly, there is a limitation for further improvement in performance, even when a larger amount of the main stream of the working fluid is supplied to flow.
In view of this fact, the turbine nozzle unit and the turbine movable blade unit, in which the throat-pitch ratio “s/t” is increased at the blade-central portion in height to ensure a larger area of the passage, cause the main stream to flow in a larger amount in a zone at the blade-central portion in height, in which the small loss occurs. It is therefore conceivable that such a structure can make further improvements in performance, thus providing advantages (see Japanese Laid-Open Patent Publication No. HEI 8-109803).
However, in the turbine nozzle unit and the turbine movable blade unit having the above-described profile, the throat-pitch ratio “s/t” is small at both of the blade-root portion and the blade-tip portion, a geometrical discharge angle “α=sin−1(s/t)”, which is calculated from the throat-pitch ratio “s/t” is also small, and a turning angle becomes large.
It is known that, when the turbine nozzle unit and the turbine movable blade unit of the axial turbine generally have the small geometrical discharge angle or the large turning angle, the boundary zone develops on the surface of the blade, thus increasing the blade profile loss.
When the flowing direction of the main stream is drastically changed in the passage between the blades, the pressure gradient from the front side “F” towards the back side “B” in the passage between the blades becomes large and the secondary flow 8 also becomes large.
In addition, fluid having a low energy, in the boundary zones on the surface of the blade, which develop in the vicinity of the blade-root portion and the blade-tip portion, as well as fluid having a low energy, in the boundary zones formed on the peripheral wall surfaces in the passage between the blades flow together with the secondary flow 8, thus constituting a factor responsible for the remarkably increased secondary flow loss.
Especially, the small throat-pitch ratio “s/t” in the blade-root portion makes the annular pitch “t” small, thus leading to a small throat “s”. The small throat “s” causes a ratio “te/s” of the thickness “te” of the rear edge in the throat “s” to become large, since it is required that the thickness “te” of the rear edge in the throat “s” has a predetermined value based on the structural requirement of the blade. As a result, the blade profile loss rapidly increases as shown in FIG. 11.
The turbine nozzle unit and the turbine movable blade unit in which the throat-pitch ratio “s/t” is increased at the blade-central portion in height, as well as the other turbine nozzle unit and the other turbine movable blade unit, which are called the “compound lean” type, any one of which have been disclosed as one of the results of the recent research, have merits and demerits as described above. It is therefore conceivable that combination of them only in their structure providing the merits, i.e., realization of a so-called “hybrid blade” makes contribution to the further improvement in the turbine stage efficiency.
An object of the present invention, which was made in view of the above-mentioned problems, is therefore to provide an axial turbine, which permits to control flow distribution of the main stream in the height direction of the blade in the passage between the blades of a turbine nozzle unit and a turbine movable nozzle and reduce the blade profile loss and the secondary flow loss at the blade-root portion, thus making a further improvement in the turbine stage efficiency.